In U.S. Pat. No. 4,823,864 there is disclosed a system for storing chemical energy utilizing aqueous absorption compositions from which water is alternately evaporated and absorbed at selected time intervals. Such an absorption system provides for energy storage during off-peak hours, typically at night, when municipal power requirements are reduced. The energy may then be selectively recovered at peak hours to produce refrigeration when the need is greatest and the demand and cost of electrical energy is highest. These systems obviously offer advantages over commonly used vapor compression refrigeration systems which do not utilize such thermal storage.
In the aforesaid patent, a number of aqueous salt systems are disclosed including alkali metal hydroxide solutions which have been found to be highly effective and efficient. Of particular interest are the aqueous solutions of sodium hydroxide, potassium hydroxide and mixtures thereof. These solutions are preferred over lithium bromide absorption solutions which are less attractive because of relatively high costs and limited energy storage capacities. The aqueous NaOH and KOH absorbent solutions offer substantial advantages over conventional ice storage systems because of their adaptability to operating at temperatures different from 32.degree. F., in particular in the range of 36.degree. F. to 50.degree. F. during the charging process with simultaneous use of heat, often available as reject heat, in the range of 80.degree. F. to 135.degree. F. which allows for the use of almost all vapor compression equipment as well as absorption equipment for charging the system. Moreover, such a system does not require the typical suction temperatures of 12.degree. F. to 22.degree. F. needed for effective ice storage and which are less efficiently achieved with vapor compression and absorption equipment. However, such advantages are practical only if adequate storage density can be obtained.
For such storage density, a heat and mass transfer additive is needed which prevents excessive subcooling in the absorption and/or desorption process while providing high accessible and usable concentration spreads between the concentrated and diluted solutions and at the same time provides high power densities thus reducing heat transfer surface requirements for a given load. However, the known heat and mass transfer additives normally used with aqueous lithium bromide absorption solutions, specifically, aliphatic alcohols such as 2-ethyl hexanol and 1-octanol, form solid precipitates when contacted with the alkali metal hydroxide solutions, and are thus unsatisfactory.